(1) Field of the Invention
The invention relates to an electron beam control method, an electron beam generating apparatus, a device using the same, and an emitter.
(2) Description of the Related Art
Electron guns in electron beam based instruments use two types of cathodes (emitters); a thermionic emitter and a field emitter. A thermionic emitter uses a tungsten filament, a pointed emitter of a single crystal or a sintered compound of lanthanium hexaboride (LaB6) or cerium hexaboride (CeB6). The emitter is heated and caused to emit thermal excited electrons to thereby generate an electron beam. A field emitter uses a sharpened conical end of an electrode on the emission side of an electron beam and emits electrons by using a tunneling effect or a Schottky effect caused by a strong electric field applied to the conical end to thereby generate an electron beam.
Note that in a case where an analysis or observation is carried out in a small region, an electron beam with a high brightness is required in order to reduce a diameter thereof (here, the “brightness” is defined as the current density per unit solid angle of the electron beam). Therefore, in recent years, a field emitter has been adopted, instead of a thermionic emitter that has been conventionally employed, in a scanning electron microscope (hereinafter also referred to as “SEM” for short) and an electron probe microanalyzer (hereinafter also referred to as “EPMA” for short) as well as other electron beam based instruments; transmission microscopy, electron beam lithography, inspection tools, etc. in analysis or observation in a small region to thereby improve a spatial resolution.
There are two types of field emitters, a cold field emitter and a thermal field emitter. In the case of a cold field emitter, the conical end of an emitter is normally made from a single crystal fine tungsten wire and is subjected to a strong electric field at room temperature whereby electrons, in the single crystal, are emitted using a tunneling effect, so that an electron beam is generated. In the case of a thermal field emitter, the conical end of an emitter made from a single crystal fine tungsten wire coated with zirconium oxide (ZrO) is heated while being subjected to a strong electric field which causes electrons to be emitted using a Schottky effect, so that an electron beam is generated. Since a thermionic emitter uses a Schottky effect as described above, it is also called a Schottky emitter.
In a Schottky emitter, a zirconium oxide layer coating the conical end of the emitter has an effect of reducing a work function of a crystal face, formed in the conical end, and which is a (100) crystal plane. Therefore, a uniform, strong electron beam is emitted and extracted from the conical end. Note that a Schottky emitter technology is disclosed in U.S. Pat. Nos. 145,042 and 145,043.
In the case of a field emitter, as described above, the current density is, however, higher than that of a thermionic emitter. In the case of a field emitter, the electron source diameter, where an electron beam is emitted from in an electron gun configuration, is very small, as shown in FIG. 9B, in comparison with a thermionic emitter of FIG. 9A (FIG. 9B shows a Schottky emitter). An electron source diameter is several tens of μm in a case of a thermionic emitter, while in a field emitter represented by a Schottky emitter, an electron source diameter is several tens of nm. If an electron source area of a thermionic emitter is indicated by dSTE, and an electron source area of a field emitter is indicated by dSFE, the areas are different from each other by up to six orders of magnitude.
On the other hand, if a solid angle of an electron beam is indicated by dΩ and a beam current value (current value is indicated by Ib), a solid angle dΩ of the electron beam varies according to a beam current value Ib to be required. If an axial brightness of the electron beam is indicated by B, a beam current value Ib is given by the following equation (1) with an electron source area dS and a solid angle dΩ.Ib=B×(dS×dΩ)  (1)
In a case where a larger beam current is necessary, it is understood from equation (1) that an effective solid angle dΩ increases for fixed brightness and source area.
A Schottky emitter is much higher in brightness than a thermionic emitter (by about three orders of magnitude). However, since an electron source area dSFE is smaller than dSTE by up to six orders of magnitude, a solid angle dΩ of an electron beam in a case where the same beam current is secured is larger in a Schottky emitter than that in a thermionic emitter. That is, if a solid angle of an electron beam in a thermionic emitter is indicated by dΩTE and a solid angle of an electron beam in a field emitter represented by a Schottky emitter is indicated by dΩFE, a relation expressed by the following equation (2) is established.dΩFE>dΩTE  (2)
That is, an angular current density which is the current per unit solid angle for a Schottky emitter is smaller than that of a thermionic emitter although the Schottky emitter has a higher axial brightness than the thermionic emitter.
Since, with a larger solid angle, an electron beam is diverged, collimation is required. As a result, in a field emitter, an aberration in an accelerating and condenser lens section downstream from the emission side exerts a large influence, so that a characteristic of the emitter, which would be by nature high in brightness, is degraded by an influence of the aberration, and the “apparent brightness” decreases as a beam current increases. FIG. 10 is a graph showing relationships between a beam current value and brightness in the case where a Schottky emitter is employed as an example of a field emitter and the case where a tungsten filament emitter is employed as an example of a thermionic emitter. The abscissa is assigned to a beam current and the ordinate is used for plotting brightness. A dotted line is a curve concerning a tungsten filament emitter and a solid line shows a curve concerning a Schottky emitter. Note that in the Schottky emitter, the curve was obtained under the conditions where the emission current density js is 1.0×104 A/cm2, an emitter temperature T is 1800 K and an angular current density JΩSE is 0.429 mA/str, while for the thermionic emitter, the curve was obtained under the conditions where the emission current density js is 3 A/cm2, an emitter temperature T is 2800 K and an angular current density JΩW=140 mA/str. The term “W filament” indicates the tungsten filament operated in the thermionic mode and the term “SE” indicates the Schottky emitter.
In a case of a thermionic emitter represented by a tungsten filament, the angular current density is high; therefore a decrease in brightness is not problematical in a practical aspect giving a reduced brightness when the current is in the neighborhood of a value in the range of 10 μA to 20 μA. On the other hand, in a case of a field emitter represented by a Schottky emitter, an angular current density is lower and an electron source diameter is smaller; therefore, the brightness begins to decrease when the beam current is in the neighborhood of 1 nA and the brightness decreases by 6 orders of magnitude at a beam current 1 μA.
Since a beam current employed in a case of a scanning electron microscope (SEM) is at a level of nA or less, no reduction in brightness is observed with a Schottky emitter in a case where the emitter is used in SEM. Thus, a Schottky emitter can be used in SEM. However, in a case of a device where a beam current at a level of sub μA or μA is required as in a electron probe micro analyzer (EPMA), reduction in the brightness is observed at a level of sub μA or μA for a Schottky emitter; therefore, even if a Schottky emitter is employed in instruments such as EPMA, only an electron beam with a low brightness can be used. Hence, it is impossible to employ a field emitter in instruments such as EPMA in a practical sense.